The present invention relates to a cartridgeholder assembly for a measured dose medication dispensing device.
Patients suffering from diabetes often have to inject themselves with frequent doses of insulin and this can be done using a conventional syringe. However, such patients often also suffer from side effects of their illness and are not capable of accurately controlling the operation of such a syringe. It is therefore desirable that they should be provided with means for automatically administering an accurately controlled dosage. The dosage required by different patients can vary over quite wide ranges, from for example 2 units of insulin per dose to 30 or more units, and it is necessary to ensure that any device is capable of selecting a range of dosages simply and accurately.
Dispensing devices such as that shown in Rex U.S. Pat. No. 4,592,745 utilize a cartridge having a pierceable membrane at one end and a movable piston at the other, with a volume of a medication such as an insulin solution contained therebetween. The cartridge is mounted in a dispensing device which includes a plunger, a one-way mechanism that permits the plunger only to advance, and a mechanism for advancing the plunger to dispense medication.
The device disclosed in the Rex patent utilizes the rear rim of the cartridge to actuate the one-way mechanism: when the cartridge is removed the one-way mechanism releases, allowing the plunger to retract, but when the dispensing device is assembled with the cartridge in place the rear rim of the cartridge causes the one-way mechanism to engage the plunger. The cartridge is received loosely in a section of the device, and the one-way mechanism engaging apparatus resiliently holds the cartridge in position.
The dispensing device of the Rex patent has been proven effective and reliable in use. Nonetheless, it suffers from certain disadvantages related to the fact that the walls of the cartridge are formed of glass and in commercially practical cartridges it is difficult to control the overall length of the cartridge accurately. Resulting variations in the length of the cartridge cause the one-way mechanism to be engaged at a variable position as the cartridge enclosing section is screwed into place in the dispensing unit. If the cartridge is unusually long, the one-way mechanism will be engaged well before the cartridge enclosing section reaches its final position, and the plunger will then pressurize the contents of the cartridge as the section is screwed home. Such pressurization will produce a squirt of medication when the needle pierces the membrane. Some users may object to this unintended release of medication.
The variable length of the cartridge also imposes design constraints on the Rex dispensing device. As mentioned above, the cartridge fits loosely within the cartridge receiving section, and the cartridge is held in position by forces applied to the rear rim of the cartridge by the engaging apparatus for the one-way mechanism discussed above. This engaging apparatus must provide resilient support to the rim over the full range of cartridge lengths. Otherwise, the cartridge may be subjected to excessive axial forces, or it may alternately be left free to move axially in the dispensing device. The resilient mounting of the engaging apparatus in no way overcomes the problems discussed above related to unintended pressurization of the cartridge.
The present invention is directed to an improved cartridge-holder assembly that overcomes these prior art problems.